Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 427
Diagnosing Secondary Shuttle Valves
Shuttle valves can cause malfunctions in hydraulic systems if they become
stuck in one position. If a technician is faced with a downstream DCV that will
not fl ow oil, it is possible that an upstream secondary shuttle valve is at fault.
For example, in Figure 18-8, if the secondary shuttle valve in DCV 1 is stuck
in the lifted position, it would cause problems downstream for DCV 2 and
DCV 3. Initially, a technician might think that DCV 2 and DCV 3 are suspect
because those are the valves having trouble fl owing oil. However, the problem-
atic shuttle valve is actually located upstream in DCV 1.
Load-Sensing System with Single-Acting Actuator
Figure 18-9 illustrates an LS system with one single-acting actuator, a uni-
directional hydraulic motor. Note that neither a primary shuttle valve nor a sec-
ondary shuttle valve is necessary. Primary shuttles are required for choosing
the higher working pressure of a double-acting actuator, for example choosing
between the working pressures at the rod end and cap end of a double-acting
cylinder. A single-acting actuator only has one possible working pressure.
In this example, it is the hydraulic motor’s forward rotation. If the hydraulic
system contained multiple single-acting actuators, secondary shuttles would
be required to choose the highest system working pressure among the multiple
actuators so it could be directed to the unloading valve.
Pressure-Compensating System
Most LS hydraulic systems are also pressure compensating (PC). The com-
bination of LS and PC can be called any one of the following names, depending
on the manufacturer:
• LSPC—Load-sensing pressure compensation system (Caterpillar).
• PCLS—Pressure compensation load-sensing system (John Deere’s
construction division).
Figure 18-9. An LS system with one single-acting actuator does not require the use of a primary shuttle valve or a
secondary shuttle valve.
Unidirectional
motor
DCV